908
FREDERICK P. XERTESS AND ROBERT C. PLUMB
Val. 67
EFFECTS OF THE XATURE OF THE SUBSTRATE UPON THE PICKUP OF ALKALISE EARTH STEARATE LAYERS1 BY FREDERICK P. MERTENS AND ROBERT C. PLUMB Department of Chenzical Engineenkg and Cherrizskry, Wor.cestei*Polytech7zic Institute, TVomester, JIassachusetts
Received October SO, 196.3 The mode of deposition of barium and calcium stearate films and the orientation of molecules within the first few layers is shown to be significantlJ-different for gold and oxidized metal surfaces such as aluminum and copper Some films may have the hydrocarbon function pointing outward, whereas others may have the carboxyl function pointing outward. Films deposited on oxidized surfaces follow a 1,3,5-sequence of pickup, whereas films deposited on gold follow a 1,2,4,6-sequence of pickup. The mode of pickup is explained in terms of specific interactions between the barium stearate molecules and the substrate surfaces.
Introduction It is well know1 from the M ork of Blodgett2and others that monomolecular films of alkaline earth salts of certain fatty acids can be formed 011 top of aqueous solutioiis containing the metal ions when a rery dilute benzene solution of the fatty acid is spread 011 the surface. These salt films may be transferred to solid substrates such as glass or metal by dipping the substrate through the film when the film is under compression. Three types of films have been observed. X type films are deposited during a down trip through the spread film, Y type films are deposited during both the down and up trips through a spread film, and Z type films are deposited during the up trip only. According to BIodgett2 the tendency to form X or Y films depends upon the pH of the solution, the temperature, and to some extent on the previous number of barium stearate monolayers which hare been deposited on the surface. Monomolecular films on water are oriented with the metal-carboxyl function pointing toward the water. The films deposited 011 glass, and presumably all other metals studied to date, hare the carboxyl heads pointed toward the substrate; the outer surface, in the first layer aiid in multilayer films, consists of closely packed CH, groups. I n Y type films the layers alternate in direction. In X type films the layers are oriented in the same direction. Attempts have been made to explain the factors promoting the formation of X, P, and Z type films. Bikermana proposed that the type of deposition depends upon the hydrophobic character of the surface. He proposed that the difference between receding and advancing contact angles and the value of the contact angle was important. Thus, depending upon the hydrophobic character of the surface, both the advancing and receding angle may be less than 90', the advancing angle may be less than 90' and the receding angle greater than 90°, or both angles may be greater than 90'. One might assume that a monolayer on a water surface could only touch and attach itself to the solid surface if the direction of movement of the slide and that of the water surface near the slide forms an obtuse angle so that the monolayer on water and the metal can approach one another. Then one would expect the tendencyfor pickup to increase with decreasing hydrophobic character (decrease in contact angles) and one might fiiid sufficient difference between advancing and receding contact angles to produce (1) Based upon a portlon of a dissertation subniitted by F. P Rlertons in partial fulfillment of the requirements for the P h D. degree i n Cherrlistry. (2) K. B. Blodgett, J . A m . Chem. Soc., 57, 1007 (1935). ( 3 ) J. J. Bikerman, Proc. Rog. SOC.(London), A170, 130 (1939).
pickup on the down trip only. In a more detailed analysis one should take account of the fact that the opposite sides of monolayers hare different hydrophobic characters and the orientation of the monolayer deposited on the previous trip through the interface 157ill hare a pronouiiced effect upon the pickup of a film. One can envision composite multilayer film pickup where the simple classification as X, Y. or Z would not be valid because of specific orienting effects of the substrate and subsequent effects of the first film upon the pickup of succeediiig films. Apparently specific effects of the substrate and the behavior of the first fern layers of a multilayer fiIm have not been the subject of much investigation. One might anticipate that there would be a definite relationship between the orientation of a deposited film and the orientation of the salt upon the surface of the mater. The early investigation of Laiigmuir and Schaefer: on enzyme films leads to the conclusion that the relative orientation of the hydrophilic and hydrophobic functions of the molecules in the deposited films cannot necessarily be predicted from their relative orientations on the aqueous surface by simply assuming that the substrate picked up the film by attaching to it the portion of the molecule in closest proximity but, rather, depends upon the hydrophilic or hydrophobic character of the substrate surface itself. Other work indicates that the attractively simple picture of a uniform monolayer is probably not correct. Epsteinj studied fatty acid monolayers by electron microscopy and electron diffraction aiid proposed the existence of groups of molecules or micelles to explaiii some of the TTarying results concerning film thickness aiid molecular orientation as obtained by prexTious workers. By and large, the majority of the significaiit work 011 barium stearate films has dealt with multilayers, rather than with the first fern monolayers close to the substrate. In the course of investigations underway in this Laboratory on the oxidation of metals, it mgs desired to make step wedges in the range of 0 to 50 A. units of thickness to test the methods which are being used for the determination of thickness of films. Several significant observations on the pickup of the first few layers were made and, in particular, effects of the nature of the substrates which had not been noted previously mere encountered. This report will describe these observations and point out certain generalities about the mode of pickup which are indicated by cur n ork. I Lungnull and T' J. Schaefer, J . Am. Chem. Soc , 60, 1361 (1938). (5) H. T Epstein, J Phys. Collocd Chem., 54, 1053 (1960).
(4)
April, 1963
? J A T U R E O F S U B S T R A T E OK P I C K U P O F ALKALINE
909
EARTH S T C A R A T E LAYERS
Experimental Preparation of Metal Substrates.-Three different met)als, aluminum, copper, and gold, were used as substrates. Clean films of these metals were prepared by evaporation i 7 ~vacuo a t approximately 3 >( 10-6 mm. pressure onto clean glass plates (10 X 70 X 1 mm.) cut from microscope slides. The plates were cleaned and vapor phase degreased with benzene before they were introduced into the vacuum system. They were ion bombarded with argon gas before the evaporation. A magnetically controlled movable shield in the vacuum chamber shielded the slides from. the volatile contaminants present in the early stages of filament heating.@ The met$ coated slides were examined with monochromat,ic light (5461 A. ) linearly polarized a t an angle of 135" t o the plane of incidence and impinging at an angle of incidence of about 70" t o establish that they were optically uniform over the entire slide surface. The conditions of evaporation were adjmted t o produce uniform films. Deposition of Stearate Layers.-The films of barium and calcium stearate were deposited by the conventional Rlodgett technique. A dilute stearic acid solution in benzene was spread on the surface of the clean aqueous solution cont.aining barium or calcium ions a t the proper pH. The stearic acid solution yas enclosed by a waxed nylon thread and the film was compressed by oleic acid. The fi1m.s were stabilized and step wedges were prepared by successive dips. 'The slides were perpendicular to t,he solution surface during dipping. The first monolayer usually required 15 to 25 minutes to dry before successive dips were made. As was observed by Blodgett, the samples ret,racted completely dry from the solution after two or three films had been deposited. Film Thickness Determinations.-The film thicknesses were measured by determining the charact,eristics of the elliptically polarized light generated by reflecting linearly polarized light from the surfaces of the step wedges. The details of this technique will not be discussed since the general procedures are already well known. Special procedures and techniques which we have developed are beinlg described elsewhere.' The film thicknesses were calculated by the Drude linear approximation for thin dielectric films on metal substrates.* The Drude linear approximation equations are valid for thin films such as were st,udied in this w o r y and their accuracy may be extended to a few per cent a t 100 A. by a modification in the calculation procedures which we are describing e1sewhe1-e.~ The explicit solution of the equat,ions for the thickness and index of refraction of any particular film was used. Computations were performed on an IBM 1620 computer.
Sequence of Film Pickup by Different Metal Surfaces.-A significant, reproducible dependence of the sequence of film pickup upon the nature of the metal surface was found. The sequence of film pickup may be readily established from experimental observations of the film thickness as a function of the number of dips by plot'ting the film thickness us. the number of monolayers for an assumed sequence of pickup and seeing which assumed sequence of pickup gives the most linear plot. Figure 1 shows such a plot for barium stearate on gold. It is apparent that the films of barium stearate are picked up by gold in a 1,2,4sequence. Similar determinations for barium stearate on copper and calcium stearate on aluminum indicate a 1,3,5-sequence of pickup in both cases. (We have also noted, under some experimental conditions, a 2,4,6-sequence on gold, but we have not studied i t in detail and will not discuss it in this paper.) Hydration of the oxide film on the surface of copper during the first dip was found to be significant. It was found, on dipping an oxidized copper film through a clean water-air interface, that a step about 14 11. thick was produced. It was assumed that this step (6) H. L. Rook and R. C. Plumb. A p p l . Phys. Letters, 1, 11 (1 9 6 9 ) . (7) F. P. Mertens and R. C. Plumb, t o be published. (8) A. Vasicek, "Optics of Thin Films," North Holland Publishing Co., 1959. (9) A. B. Winterbottom, Korske Pidenskabers Selskab, 46, 1 (1955).
100 I
i
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I st dip 0 2 n d dip
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1 2 3 4 5 6 7 NUMBER OF M O N O L A Y E R S ,
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Fig. 1.--Observed thicknesses of barium stearate films on a gold substrate plotted for a 1,2,4- and a 1,3,5-sequence of pickup.
was the boundary betn-een hydrated copper oxide and the unhydrated oxide. The observed film thicknesses on copper mere corrected for the hydration of the oxide. The film thickness, plotted as the ordinates in Fig. 1, is in arbitrary units. Absolute values for the film thicknesses are being published elsewhere along with a more comprehensive discussion of the problem of thin film thickness determinati~n.~ From the work of Blodgett and others it is well established that, under these conditions, no film is picked up on the first down dip, one film is picked up on the first up dip, and a film is picked up on each of the subsequent down and up dips producing a 1,3,5sequence of film deposition. The sequence of film pickup could be observed o n copper and aluminum by observing the motion of the thread boundary during the down and up trips. An unsuccessful attempt was made to decide if the films deposited in the first two dips on gold were deposited on the down trip or the up trip. Glass microscope slides mere coated with gold by two evaporations, one 011 each side. The changes in area of the spread barium stearate monolayer on mater during dipping were measured by photographing the thread. A confusing pattern of results emerged which did not clearly indicate deposition on either the down trip or the up trip. It was found when the two sides of the gold coated slide were examined with elliptically polarized light that film deposition was different on the two sides. One side gave a deposition characteristic of gold, whereas the other side gave a deposition in a 1,3,5-sequence, characteristic of other metals. It is believed that the surface of the gold which gave the nongold like behavior was, in fact, contaminated with a, foreign metal as a result of its having deposited first in the vacuum system. Two possible mechanisms by
AND ROBERT C. PLUMB FREDERICK P. UIERTENS
910 ID
IU
2D
2u
20
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PICKUP I N 1,3,5 SEQUENCE
c. ID
PICKUP I N 1,2,4
SEQUENCE
Fig. 2.-Schematic representation of process of film pickup in 1,3,5-sequence and l12,4-sequence.
which it may have become contaminated are diffusion of impurities to the surface (which would be enhanced by the ion bombardment used to clean the glass on the other side of the slide prior to evaporation) and sputtering of aluminum from the aluminum ion bombardment electrodes. Orientation of Molecules within the Deposited Films. -It is to be expected from the work of others that, in the l13,5-sequenceof film formation, the hydrocarbon function points outward in the first, third, and fifth layers. By forming step wedges consisting of 1,3,5 and 7 layer steps and observing the flow characteristics of drops of benzene placed upon these steps, it \vas established that all of the outer surfaces had approximately the same hydrophobic character. The contact angle to benzene and the tendency of benzene drops to flow when placed a t one end of an incline step m-edge showed no differences between successive steps. A variety of orientations are conceivable in the 1,2,4,6-sequence of film pickup observed on gold. On the 2, 4, and 6 layer steps benzene behaved the same as on the 1,3,5-steps observed on other metals. The benzene behaved markedly different on the 1 layer step adjacent to the metal surface. The benzene wet this layer readily and flowed up to the second step where there was a pronounced barrier. Thus, in the 1,2,4,6sequence the first layer has the hydrocarbon function pointed toward the gold surface and the carboxylate group pointed outward, the second layer has the carboxyl group pointed toward the carboxyl group of the first layer and the hydrocarbon group outward, and succeeding layers are in the same orientation as that observed on aluminum and copper. Discussion The essential differences between gold and aluminum and copper seem to be caused by the fact that both aluminum and copper are coated with oxides, whereas the gold is, to a great extent, oxide-free. It has been demonstrated that the unexpected observation of oxidation of gold by some investigators really results from diffusion of impurities in gold to the surface forming an impurity oxide film.I0 Apparently a pure gold surface is essentially oxide-free.
Vol. Gpf
It has been shown by Langmuir and Schaefer that the orientation of layers in a multi-layer film is not, necessarily that which one would predict from the directioti of traverse when the film is being picked up. There is no fundamental reason to expect that molecules a t the air-substrate-film interface cannot rotate as they are being absorbed. The explanation of the mode of pickup and the orientation of molecules which are picked up must lie in specific interactions between the various substances involved. We will assume, as seems reasonable, that carboxyl functions may interact with each other, if in the right orientation, by dipole-dipole forces. The principal iizteraction between hydrocarbon groups will be by Heitler-London Dispersion forces. The interactions between carboxyl groups and hydrocarbon groups will be of a mixed Heitler-London Dispersion and dipoIeinduced dipole character. The attractive energy between two carboxyl groups or between two hydrocarbon groups will be assumed to be stronger than the energy of interaction betn-een a carboxyl group and a hydrocarbon group. The process by which layers are deposited and specific orientations of the molecules produced, as we envision it, is shown schematically in Fig. 2. Tt’e believe that pickup of the first layer in a l12,4-sequencetakes place when the slide is being withdrawn from the solution, but we are not completely certain about this point. This pickup would involve a reorientation of molecules a t the three-phase interface. The second down trip of the slide may or may not pick up a film depending upon the orientation of the first layer which has already been deposited. If the hydrocarbon function of the first layer is pointed outward, then it will be energetically favorable to deposit a second layer because, by so doing, the molecules avoid generating a high energy hydrocarbon-solution interface. The second down dip in a 1,2,4-sequence will not produce pickup because of the weak attractive forces between the hydrocarbon function and the carboxyl group. Pickup of a second layer a t this stage by attraction of the carboxyl group of one molecule to the carboxyl group of another mould require generation of a hydrocarbon-water interface which would be unstable. After the second down trip the outer surfaces of either a l13,5-sequence or a 1,2,4-sequence are identical and the films mill be picked up in the usual way. Apparently the carboxyl groups are strongly attracted to oxides, whereas the hydrocarbon function is attracted to a bare metal surface. Thus, one would classify a bare metal as more hydrophobic than an oxide coated metal. According to this picture then the orientation of the first layer is determined by the specific interactions between the barium stearate film and the metal surface. The orientation of the second layer is determined by specific interaction between the molecules in the second layer and molecules in the first layer. Acknowledgment.-This research was supported by United States Atomic Energy Commission Basic Research Contract AT(30-1)2479. (10) L. D. Carpenter, D. Clark, W. II. Main, and T. Dickinson, T i m a . Faraday Sac., 66, 1924 (1969).
